I think it is entirely up to mom & dad (once coach makes the suggestion). They know their children best.

If the child & family can handle the hours (sometimes hours are longer & more often), bigger commitment etc., more travel, more money, then it is great. Now, on the other hand many parents appreciate the less pressure, less hours of L4. - this is great to. Totally depends on the child & family preference.
Not all USAG gyms compete L4, ours did not so my dd had all of her L5 skills when she was 5, and on pre-team. She only recently turned 7, & will compete her 1st season this year as a L5, and then move onto L6. I believe emotionally & physically she is 100% ready for the challenge. Try not to look at gymnastics in terms of levels... because children learn skills at a different pace. Sometimes they fly through everything, then stall out, and some have a slower approach, and then fly through skills when their confidence improves, but at some point they eventually - if the child stays in the sport - all catch up to each other. Neither path is wrong... The important thing at this age is to keep it fun, and - I emphasize - as pressure free as possible.
Also, keep in mind that a 6 yr old will need to wait until the DAY they turn 7 yrs old to be age eligible to compete. This worked best for us, but I know plenty of kids/parents that were thrilled & excited at the chance to compete when they were 6 yrs old (L4), and would not have traded it for anything. For our dd, I felt there was no pressure from competing and my dd could really enjoy the preperation - without any of the nerves. Although, she had to watch all of her teammates compete while she sat on the sidelines...
Best of luck
